Due to the increasing energy demand, offshore oil and gas production is moving into deeper waters. Wells for hydrocarbon production can be located in water depths down to 3,000 m or more and a considerable distance from shore. For ensuring an efficient and secure production, processing facilities are being installed at the ocean floor in proximity to such subsea well. Installations on the ocean floor can comprise a range of components, including pumps, compressors and the like which require electric power for operation. Power supply can occur by way of a subsea power grid installed on the ocean floor, which may for example comprise a subsea transformer, a subsea switchgear, and a subsea variable speed drive (VSD) for powering subsea loads.
In order to be capable of powering the subsea installation, voltage is supplied at a high voltage level to the subsea transformer and then transformed to a distribution voltage level, which may for example be about 36 kV. In such example, the switchgear provides 36 kV distribution and switching, which has to occur by means of vacuum circuit breakers. In particular, the switchgear is required to be capable of breaking a short circuit current so that in case of a fault in a subsea load, the faulty load can be disconnected from the subsea power grid. The cables and connectors used between the different components of the subsea installation are rated for the distribution voltage, e.g. for the above-mentioned 36 kV.
When installed subsea, the components of the subsea power grid are exposed to pressures up to or even in excess of 300 bar (at a depth of 3,000 m). To protect the components from the corrosive environment of the surrounding seawater and to deal with the high pressures, pressure resistant enclosures or pressure compensated enclosures can be used. In particular for vacuum circuit breakers provided in a switchgear, pressure resistant enclosures need to be used. They need to have thick walls to withstand the high differential pressure between the subsea environment and the inside of the enclosure.
Accordingly, the switchgear, as well as the other modules such as subsea transformer or variable speed drive are relatively heavy and large sized. Furthermore, they are relatively expensive to produce. In particular for smaller loads, the supply with electric power via components of such subsea power grid might be expensive, and might require these relatively big and heavy modules.
To power such small individual consumers, it is known to provide a single power supply cable, for example from a topside installation, such as an offshore platform or an onshore facility. Although such solution does not require the relatively expensive and heavy subsea components, it requires a subsea cable or an umbilical for each load, which, depending on the length, can also be expensive and difficult to install.
It is desirable to be capable of selectively powering relatively small individual consumers of a subsea installation, which can be located at different positions subsea, without the need for such cables or the above mentioned expensive and heavy subsea components.